1. Field of the Invention
This invention relates to an optical recording method for forming a groove on a writable or rewritable optical information recording medium.
2. Description of the Prior Art
Referring to a block diagram of FIG. 1 showing a conventional optical recording apparatus, a laser light source 1 emits an Argon laser beam of a Krypton laser beam, and an optical light beam deflector 2 provides a zeroth order light beam L.sub.o and an nth order diffracted light beam L.sub.n (n=1, 2, . . . ) having an angle of diffraction that corresponds to an output of an amplifier 14.
The optical light beam deflector 2 is a so-called A/O deflector utilizing the acousto-optic effect. Ordinarily, an nth order diffracted light beam L.sub.n to be selected for writing is a first order diffracted light beam L.sub.1. A recording optical system 7 includes a mirror 7a for reflecting the first order diffracted light beam L.sub.1 derived from the optical light beam deflector 2, and an objective lens 7b for converging the first order diffracted light beam L.sub.1 reflected by the mirror 7a onto a recording surface of a recording disk R as a write beam spot. A driving unit 8 includes a turntable 8a mounted with the recording disk R, a motor 8b for rotating the turntable 8a, a motor driving circuit 8c for driving the motor 8b, a feed mechanism 8d for feeding the recording optical system 7 in a direction radial to the axis of the recording disk R, and a reference signal generator 8e for supplying reference signals to the motor driving circuit 8c and the feed mechanism 8d. A first signal generator 9 generates a carrier wave f.sub.c for determining an angle of diffraction of an nth order diffracted light beam L.sub.n coming from the optical light beam deflector 2. A gate signal generator 12 generates a gate signal S for controlling a gate circuit 13. The gate circuit 13 passes the carrier wave f.sub.c fed by the first signal generator 9 to an amplifier 14 while the gate signal generator 12 generates the gate signal S of ON. The amplifier 14 amplifies the carrier wave f.sub.c fed by the first signal generator 9 and applies the amplified carrier wave f.sub.c to the optical light beam deflector 2.
FIGS. 2A, 2B and 2C are digrams for illustrating the function of the optical recording apparatus of FIG. 1 to form a guide groove on a surface of the recording disk. As shown in FIG. 2C, the recording disk R consists of a substrate B and a photoresist film C of a thickness on the order of 1000 .ANG. formed over the major surface of the substrate B. The photoresist film C is exposed to a light beam having a spot L.sub.s of a diameter d.sub.11 to form the groove G thereon.
In operation, the laser light source 1 emits a light beam L, the first signal generator 9 supplies a carrier wave f.sub.c to the amplifier 14. The amplifier 14 applies the carrier wave f.sub.c to the optical light beam deflector 2 after amplification. Then, the optical light beam deflector 2 derives a zeroth order light beam L.sub.0 and an nth order diffracted light beam L.sub.n therefrom. Further, the first order diffracted light beam L.sub.1 coming from the optical light beam deflector 2 is reflected by the mirror 7a in the recording optical system 7 and is projected onto the photoresist film C of the recording disk R through the objective lens 7b. While the first order diffracted light beam L.sub.1 is being projected onto the photoresist film C, it is possible to rotate the turntable 8a mounted with the recording disk R and then to transfer the recording optical system 7 in a direction radial to the axis of the recording disk R synchronously with the rotation thereof by supplying the reference signal generated by the reference signal generator 8e in the driving unit 8 to the motor driving circuit 8c and the feed mechanism 8 d.
FIG. 2A shows the Gaussian distribution of intensity of the first order diffracted light beam L.sub.1 passed through the objective lens 7b in projecting the first order diffracted light beam L.sub.1 onto the photoresist film C. Accordingly, the diameter d.sub.11 of the light spot L.sub.s (FIG. 2B) converged on the photoresist film C is equal to the diameter of a portion of the light spot L.sub.s where is 1/e.sup.2 times the peak value of the Gaussian distribution of intensity of the light beam shown in FIG. 2A. When it is assumed that the light beam L is an Argon laser beam of 458 nm in wavelength, the numerical aperture NA of the objective lens 7b is 0.9, a width of a groove G to be formed by the light spot L.sub.s may be varied within a range of approximately 0.3 .mu.m to 0.8 .mu.m by adjusting the diameter of the light beam L emitted by the laser light source 1 through the change of light beam power.
However, in the optical recording apparatus of FIG. 1, since it is unable to form a groove G of width at or greater than 0.8 .mu.m when the numerical aperture NA of the objective lens 7a is 0.9, there have been proposed several methods to form a wide groove G having a width greater than 0.8 .mu.m. One of the methods implemented by the optical recording apparatus shown in FIG. 1 will be described with reference to FIGS. 3A, 3B and 3C. To carry out the method for forming a wide groove G, the effective numerical aperture NA' of the objective lens 7b of the recording optical system 7 is reduced more or less, and thereby the diameter d.sub.12 of the light spot L.sub.s (FIG. 3B) converged on the photoresist film C will be the same diameter of a portion which is 1/e.sup.2 times the peak value of the Gaussian distribution of intensity of the light beam as shown in FIG. 3A, thus resulting in a wide groove. That is, when the light beam L is an Argon laser beam of 458 nm in wavelength and the effective numerical aperture NA' of the objective lens 7b is reduced to 0.45, the groove G having a width of approximately 0.5 .mu.m to 1.5 .mu.m can be formed in the photoresist film C by the light spot L.sub.s by adjusting the diameter of the light beam L emitted by the laser light source 1 through the change of the beam power.
Another conventional optical recording apparatus will be described hereinafter with reference to FIG. 4, wherein like numerals denote like elements in FIG. 1 and the description thereof will be omitted.
Referring to FIG. 4, a first beam splitter 3, such as of a half mirror, splits a first order diffracted light beam L.sub.1 coming from the optical light beam deflector 2. A first mirror 4 reflects the first order diffracted light beam L.sub.1 passed through the first beam splitter 3 towards a second beam splitter 6, such as of a half mirror. A second mirror 5 reflects the first order diffracted light beam L.sub.1 reflected by the first beam splitter 3 towards the second beam splitter 6. The second beam splitter 6 reflects the first order diffracted light beam L.sub.1 coming from the first mirror 4, and transmits the first order diffracted light beam L.sub.1 coming from the second mirror 5. The first beam splitter 3, the second beam splitter 6, the first mirror 4 and the second mirror 5 are arranged so that the center distance between the first order diffracted light beam L.sub.1 transmitted through the second beam splitter 6 and the first order light beam L.sub.1 reflected by the second beam splitter 6 corresponds to the width of a groove G to be formed.
The groove forming steps of the optical recording apparatus shown in FIG. 4 will be described with reference to FIGS. 5A, 5B and 5C. In this case, an elliptic spot L.sub.s is formed on the photoresist film C. A groove G is formed in a direction along the minor axis of the elliptic spot L.sub.s. As stated above, the first order diffracted light beam L.sub.1 coming from the optical light beam deflector 2 and passed through two light paths, one of which is formed by the first beam splitter 3, the first mirror 4 and the second beam splitter 6, and the other of which is formed by the first beam splitter 3, the second mirror 5 and the second beam splitter 6, to the recording optical system 7 has the Gaussian distribution of intensity as shown in FIG. 5A. Accordingly, the spot diameter d.sub.13 of the light spot L.sub.s (FIG. 5B) concentrated on the photoresist film C is 1/e.sup.2 times the peak value of the Gaussian distribution of intensity shown in FIG. 5A. Therefore, when the light beam is the Argon laser beam of 458 nm in wavelength and the numerical aperture NA of the objective lens 7b is 0.9, a groove G having a width in the range of approximately 0.3 .mu.m to 1.5 .mu.m can be formed by adjusting the diameter fo the light beam emitted by the laser light source 1 and the center distance between the first order diffracted light beams L.sub.1 passed through different light paths through the change of beam power.
A groove G having an increased width can be formed by the conventional optical recording apparatuses as described above. However, when a groove G is formed in the manner as illustrated in FIGS. 3A, 3B and 3C, since the gradient at a portion of the Gaussian distribution of intensity which determines the spot diameter d.sub.12 is dull as shown in FIG. 3B, the groove G may have rounded edges as shown in FIG. 3C, which in turn deteriorates the recording and reproducing characteristics of the groove G.
Further, when the groove G is formed in the manner as illustrated in FIGS. 5A, 5B and 5C, since the gradient at a portion of the Gaussian distribution of intensity which determines the spot diameter d.sub.13 is steep, the groove may have sharp edges as shown in FIG. 5C, which improves the recording and reproducing characteristics fo the groove G. However, it is difficult to align the optical system of the first beam splitter 3, the second beam splitter 6, the first mirror 4 and the second mirror 5 with the groove having the required width.
It is therefore an object of this invention to eliminate the problems encountered in the optical recording apparatus of prior art and to provide an optical recording method capable of forming a relatively wide groove having satisfactory characteristics in recording and reproducing.